In co-pending application Ser. No. 732,637 filed May 10, 1985, the disclosure of which is incorporated by reference herein, procedures and apparatus for efficiently recovering gold and/or silver from ores utilizing oxygen gas (instead of normal air) in a cyanide leaching-adsorption process are disclosed. In such procedures, typically a plurality of tanks are interconnected together, each tank having a slurry inlet and a slurry outlet, with a screen surrounding the slurry outlet. The screen functions to screen the activated charcoal (carbon particles out so that they do not pass with the slurry into the slurry outlet. The inventory of charcoal particles is transferred from one vessel to the next in the opposite direction of the flow of pulp during treatment, with carbon discharged from the first vessel in the series ultimately being passed to a gold and/or silver recovery station, while the pulp discharged from the last vessel in the series is leach residue, which can be disposed of.
Existing carbon-in-pulp (CIP) and carbon-in-leach (CIL) process experience operating difficulties due to plugging of the carbon screens, however. Normally, the material which plugs the screens is not the charcoal particles, but rather stringy material, such as extraneous wood and plastic which has been thoroughly beaten and softened by passage through a ball mill. In normal operations, maintenance workers clean the screens by hand, and this is a simple procedure that may not require shutdown. However in CIL and like systems which utilize oxygen instead of normal air to facilitate leaching, such as shown in said co-pending application Ser. No. 732,637, cleaning the screens by hand would be difficult and dangerous. The tank covers required to maintain the oxygen environment would be in the way, and working in high oxygen content environments can be risky. Therefore it is desirable to provide some means of self-cleaning the screens in such oxygen CIL and CIP processes.
According to one aspect of the present invention, apparatus is provided for use in separating metal, particular gold and/or silver, and other values from a slurry, preferably a cyanide slurry of gold and/or silver ore particles, by adsorption with particles, preferably activated charcoal (i.e. carbon) granules. The apparatus includes a tank for holding the slurry containing the values (precious metal) to be separated, and a number of particles (carbon) on which the values are to be adsorbed, the tank having a slurry inlet and a slurry outlet. According to the present invention, a screen surrounds the slurry outlet, and means are provided for linearly reciprocating the screen to provide a self-cleaning action thereof due to back flushing of the screen. The screen is moved slowly in a first direction, and at the end of travel is then moved very quickly in the opposite direction. During movement in the second direction, the back flushing action occurs. If necessary or desirable, gas relief means can be provided, and also the screen is preferably tapered in the direction of movement. By providing the tapered screen, it is possible to utilize the screen to additionally transport and concentrate charcoal particles at the end of travel of the screen in the first direction thereof. A suction pipe, or the like, for recirculating the carbon particles to other tanks may be provided at the area of concentrated particles adjacent the first end of travel of the screen so that the carbon flow can be minimized. The screen may have a wide variety of configurations, such as a conical configuration, and bearings for guiding the reciprocation can be provided at one or both ends thereof.
According to another aspect of the present invention, the tank includes a vertically extending draft tube which has an open bottom and open top. A shaft rotatable about a vertical axis extends through the top of the tank, and has agitator blades disposed above the draft tube. The screen for the slurry outlet preferably comprises a hollow cylindrical body which is concentric with the shaft, and interiorly of the screen a screen cleaning rotor is connected to the shaft. In that way, as the shaft rotates circulation of slurry and/or particles within the tank is effected by the slurry being drawn upwardly through the draft tube, and the screen is continuously kept free of clogging materials. The rotor may comprise a plurality of generally vertically extending circumferentially spaced blades, or a screw.
According to another aspect of the present invention, a process for effecting the recovery of gold and/or silver from a slurry of ore or the like is provided. The process includes the conventional steps of leaching gold and/or silver from the ore utilizing a basic cyanide solution, and recovering the leached gold and/or silver in solution by contacting the slurry with solid material adsorbing particles (e.g. charcoal particles). The recovering step is preferably practiced by providing oxygen gas in the slurry in an amount significantly greater than can be obtained by contacting the slurry with air, as disclosed in said co-pending application Ser. No. 732,637, and additionally the process also includes the step of back flushing the screen surrounding the slurry outlet of each tank in the recovery process, to keep the screens clean. The back flushing can be accomplished utilizing the apparatus described above.
It is the primary object of the present invention to provide an apparatus and process facilitating CIP and CIL processes utilizing oxygen gas, and screens surrounding tank slurry outlets. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.